Proteins of the innate immune system known as the alternative pathway of complement have recently been linked to approximately 74% of the cases of age-related macular degeneration (ARMD). Several years ago it was genetically linked to inherited hemolytic uremic syndrome (aHUS). Both of these diseases are associated with mutations in the complement protein factor H. Factor H is the primary regulator of alternative pathway activation. A common allele of factor B has been shown to be protective for ARMD. Factor B is central to activation of the alternative pathway and its activation is controlled by factor H. ARMD is the leading cause of blindness in the developed countries and 30% of people over the age of 65 have ARMD or show early signs of the disease. Much progress toward understanding the functions of factor H and factor B has been made in the previous grant. We have determined the NMR-derived 3D structure of the two domains in factor H involved in aHUS and characterized the function of these domains. Extensive studies of the C5-activating enzyme formed by factor B have also been completed. The present grant proposes to continue this work now focusing on ARMD-related structures and their functional consequences in complement activation. Activation of the alternative pathway is capable of inflicting great damage to host tissues and has been associated with many diseases. The proposed research will investigate disease-associated alleles in factor H and identify and characterize what now appear to be tissue specific recognition sites in the 20 domains of factor H. Novel approaches have recently been developed in our lab to characterize the tissue-specific receptors for these sites and to examine the functional role of each site. Several of these sites have already been shown to be critical for regulation of complement activation on host cells and tissues. Alleles of factor B will also be examined and their functional consequences evaluated to understand the mechanism by which these alterations provide protection from macular degeneration. Relevance: The benefits of this research will be a better understanding of the molecular pathology of inherited hemolytic uremic syndrome and of age-related macular degeneration (ARMD). Recent discoveries point to the alternative pathway of complement as a major contributor to these diseases. This proposal will examine the molecular events leading to tissue damage in these and other diseases.